With the spread of large display devices such as liquid crystal televisions, display devices have needed higher added value and developed. In particular, a technique for forming a driver circuit such as a scan line driver circuit over the same substrate as a pixel portion with the use of a thin film transistor (TFT) whose channel region is formed using an amorphous semiconductor (especially, an oxide semiconductor) has actively developed.
A thin film transistor whose channel region is formed using an amorphous semiconductor is often used for a driver circuit formed using only n-channel transistors or p-channel transistors. For example, a structure disclosed in Reference 1 can be given.